O6-Alkylguanine-DNA-alkyltransferase (AGT) is a DNA repair protein. AGT removes alkyl and aralkyl groups that become attached at the O6-position of guanine in DNA following exposure to mutagenic and/or carcinogenic alkylating agents. It does so by bringing about a stoichiometric transfer of the group attached to the O6-position of a guanine residue in DNA to a cysteine residue within the AGT protein (Pegg, Cancer Research, 50: 6119-6129 (1990)). Accordingly, AGT is beneficial to a normal cell because it removes the adducts that are formed in DNA by toxic, mutagenic and carcinogenic agents, thereby restoring the DNA to its original state and helping to prevent DNA mutations that can lead to initiation of tumor formation. Unfortunately, AGT is also beneficial to a cancerous cell because it also removes those adducts that are formed at the O6-position of guanine in DNA by antineoplastic alkylating agents, such as monofunctional methylating agents, e.g., procarbazine, dacarbazine and temozolomide, and chloroethylating agents, i.e., chloroethylnitrosoureas (CENUs), such as BCNU, ACNU, CCNU, and MeCCNU (Pegg et al., Prog. Nucleic Acid Research Molec. Biol., 51: 167-223 (1995)). The resulting alkylated AGT molecule is consequently inactivated and is unable to carry out subsequent dealkylation reactions. The presence of more AGT in a cell increases its capacity to repair DNA by this mechanism compared to a cell that has less AGT.
The reduction in the efficacy of cancer chemotherapeutic drugs due to AGT, which acts without requiring the presence of additional enzymes or cofactors, and the existence of a high correlation between AGT activity and reduction in sensitivity of tumor cells to nitrosoureas have led to AGT becoming a prime target for modulation. Modulation has been attempted by two different routes. One route is indirect and involves the use of methylating agents that introduce O6-methylguanine lesions into DNA for subsequent repair by AGT, thereby depleting levels of AGT. The other route is direct and involves the use of an adjuvant, i.e., an inactivator of AGT, such as an O6-aralkylguanine, e.g., O6-benzylguanine; see, for example, Moschel et al., U.S. Pat. Nos. 5,091,430; 5,352,669; 5,358,952; 5,525,606; 5,691,307; 5,753,668; 5,916,894; 5,958,932; 6,060,458; 6,172,070; 6,303,604; 6,333,331; and 6,436,945. It has been shown that such adjuvants can inactivate AGT and that this inactivation can markedly improve the effectiveness of chemotherapeutic drugs that modify the O6-position of DNA guanine residues (Pegg et al., Prog. Nucleic Acid Res. Mol. Biol., 51: 167-223 (1995); Kokkinakis et al., Clin. Cancer Res., 5: 3676-3681 (1999); Dolan et al., Biochem. Pharmacol., 46: 285-290 (1993); Felker et al., Cancer Chemo. Pharmacol., 32: 471-476 (1993); Schold, Jr. et al., Cancer Res., 56: 2076-2081 (1996); and Kurpad et al., Cancer Chemo. Pharmacol., 39: 307-316 (1997)). In some instances, however, in clinical trials, the adjuvant therapy produces toxic side effects in the patient (Quinn et al., J. Clin. Oncol., 2002, 20, 2277-2283). Thus, there exists a desire for compounds that are selective to the tumor cell or have reduced toxic side effects in adjuvant therapy.